The present application is based on and claims priority from Japanese Patent Application 2000-325603 filed Oct. 25, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a revolving field type AC generator and, particularly, a rotor that has a plurality of circumferentially disposed claw poles.
2. Description of the Related Art
Recently, as various efforts to reduce vehicle running resistance such as shaping the vehicle front into a slant nose and to increase the passenger compartment of a vehicle have been made, the engine compartment of the vehicle has become narrower and narrower. Accordingly, there is only a limited room for an AC generator. Such an AC generator has to be driven at a lower speed in order to reduce fuel consumption of an engine. On the other hand, various safety devices have been installed into a vehicle, and the AC generator has been required to increase electric power. In other words, a more compact and more powerful AC generator has been demanded.
In order to fulfill this demand, various improvements on a rotor and a stator have been proposed. For example, a plurality of permanent magnets is inserted between pole pieces of a rotor to supplement effective magnetic flux, thereby increasing output power. For this purpose, the permanent magnets have to have precise dimensions to be assembled, resulting in that the manufacturing cost of the vehicle AC generator increases.
In order to solve the above problem, a magnet holder for holding a permanent magnet that allows variation in distances between pole pieces or a resinous permanent magnet that is easy to fill between the pole pieces were proposed. However, the magnet holder is rather complicated in shape, and the manufacturing cost thereof can not become lower. Because the shape of the resinous permanent magnet after being filled becomes also complicated, it is very difficult to form magnetic orientation that provides strong magnetic force. In addition, since it is difficult to bond the pole pieces and the permanent magnet tightly, the permanent magnet may fly off.
In view of the above problem, an object of the invention is to provide an AC generator that can be manufactured more easily at a lower cost.
For this purpose, a cylindrical permanent magnet is provided to supplement magnetic fields of the claw poles.
Another object of the invention is to provide a more reliable AC generator. In addition, another object of the invention is to provide a compact and powerful AC generator.
According to a feature of the invention, a cylindrical permanent magnet is disposed inside a plurality of claw poles to have the magnetic fields as the claw poles when the field coil is energized. Inner surfaces of the claw poles contact the outer periphery of the cylindrical permanent magnet so that centrifugal force of the cylindrical permanent magnet is supported by the claw poles. Therefore, a reliable AC generator can be provided.
In addition, because the cylindrical permanent magnet is magnetized to have the same polarity as the claw poles when the claw poles are excited by the field coil, a stator coil is supplied with magnetic flux from both the cylindrical permanent magnet and the claw poles, so that the output power can be increased to the same extent. Further, because of the simple cylindrical shape, the orientation of the cylindrical permanent magnet can be made easy so that strong permanent magnet can be provided.
According to another feature of the invention, two kinds of cooling fans are fixed to opposite axial ends of the pole core. Therefore, axial cooling airflow is generated due to a pressure difference in the axial direction. Because a gap is formed between the cylindrical permanent magnet and the field coil, the cylindrical permanent magnet and the field coil can be cooled effectively, so that effective magnetic fluxes of the cylindrical permanent magnet, which has a negative temperature characteristic, and of the field coil can be increased.
According to another feature of the invention, a magnetic member covers the inner surface of the cylindrical permanent magnet. Therefore, the cylindrical permanent magnet is reinforced to sufficiently withstand the centrifugal force. Further, the permeance of the cylindrical permanent magnet can be improved so that the cylindrical permanent magnet can be prevented from being demagnetized by the counter magnetic field of the field coil.
According to another feature of the invention, a non-magnetic member covers an outer periphery of the cylindrical permanent magnet. Therefore, even if the cylindrical permanent magnet is broken, the cylindrical permanent magnet is prevented from scattering. In addition, the non-magnetic member protects the cylindrical permanent magnet from foreign materials, so that a reliable AC generator can be provided.